This invention pertains to polyester resins, more specifically thermoset polyester resins used for gel coats.
Gel coats have been known in the field of fiber reinforced polymers. Typically, the gel coat resin is dissolved in a reactive monomer such as styrene or methyl methacrylate (MMA) in order to apply the gel coat as a spray. Depending on the spray conditions, formula, equipment and a number of other factors, roughly a third to more than half of the styrene and MMA in a gel coat formula can be lost to the atmosphere during application. For instance, based on the CFA""s Styrene Emissions Determination Models, a single 540 pound drum of a conventional gel coat, when applied by uncontrolled spray techniques, could generate over 100 pounds of emissions known as VOC (volatile organic compounds). A fabricator using two drums of gel coat per day will generate over 25 tons of VOC emissions in one year.
Government regulations, such as the 1990 Clean Air Act (Section 112) regulate the emissions for a number of products and processes including the emissions of hazardous air pollutants (HAP) resulting from the spray application of gel coats during the open molding process. The EPA sets MACT (Maximum Achievable Control Technology) standards that industries must follow. As such, fabricators and resin suppliers are interested in lower VOC emitting gel coats.
Another important aspect of gel coats is the tackiness and cure time of the gel coat. Tackiness affects how soon the fiber-reinforced polymer can be laid up in the mold and, for a large open mold such as boat hulls, how soon the gel coat can be walked on. Many prior art gel coats are still tacky after more than one to two hours of curing.
Therefore, it would be desirable to have a low VOC low tack gel coat that solves the aforementioned problems.
The present invention provides a low VOC low tack gel coat that overcomes the aforementioned problems.
In one preferred embodiment, the invention is a low VOC, low tack gel coat composition comprising: (A) a unsaturated polyester resin derived from at least two reactants, one reactant of which is maleic anhydride, the resin characterized by: (a) a high shear viscosity (at least 500 sxe2x88x921) in styrene solution, at a concentration of 70% non-volatile matter, less than or equal to about 1750 cps; (b) a glass transition temperature greater than or equal to about 11xc2x0 C.; and, (c) a weight average molecular weight, Mw, less than or equal to about 6,050; and, (B) up to 34% by weight, based on the total weight of the composition, of an organic solvent capable of polymerization with the unsaturated polyester resin.
Another preferred embodiment of the invention is a low VOC, low tack gel coat composition comprising: (A) a unsaturated polyester resin obtained by (1) reacting a first reaction mixture comprising a dicarboxylic acid and at least one polyol to form a first reaction product, and (2) reacting a second reaction mixture comprising the first reaction product and further reactant monomers comprising maleic anhydride and a monocarboxylic acid to form the unsaturated polyester resin; and (B) up to 70% by weight of the total composition of an organic solvent capable of polymerizing with the unsaturated polyester resin, wherein the unsaturated polyester resin is characterized by: (a) a high shear viscosity (at least 500 sxe2x88x921) in styrene solution, at a concentration of 70% non-volatile matter, less than or equal to about 1750 cp; (b) a glass transition temperature greater than or equal to about 11xc2x0 C.: and, (c) a weight average molecular weight, Mw, less than or equal to about 6,050.
In still another preferred embodiment, the invention is a process for preparing a low VOC, low tack gel coat, the process comprising the steps of: (A) forming a first reaction mixture comprising a dicarboxylic acid and at least one polyol; (B) reacting the first reaction mixture to form a first reaction product; (C) forming a second reaction mixture comprising the first reaction product and further reactant monomers comprising maleic anhydride and a monocarboxylic acid; and (D) reacting the second reaction mixture to form a unsaturated polyester resin, wherein the unsaturated polyester resin is characterized by: (a) a high shear viscosity (at least 500 sxe2x88x921) in styrene solution, at a concentration of 70% non-volatile matter, less than or equal to about 1750 cp; (b) a glass transition temperature greater than or equal to about 11xc2x0 C.; and, (c) a weight average molecular weight, Mw, less than or equal to about 6,050.